This invention relates to thermoset composites containing a thermoplastic resin and, more particularly, is concerned with a process for increasing damage tolerance in thermoset composites without loss of processibility or mechanical properties, including thermal stability, by incorporating a thermoplastic filler and controlling the solubility of such thermoplastic material in the thermoset.
Current high temperature thermosets or thermosetting resins exhibit either thermal stability and poor damage tolerance or damage tolerance and poor thermal stability. The term "damage tolerance" as employed herein is intended to denote the ability of a material, such as a thermoset composite to withstand impacts, flaws, holes or other damages to the composite without propagating the damage or causing undue mechanical property loss as a result of such damage.
There has been considerable effort made recently to improve damage tolerance in thermoset matrix resins, particularly in the production of damage tolerant aircraft structures. Epoxy and BMI (bismaleimide) matrix systems are too brittle for primary structures Many epoxy-based resin systems have been toughened by the use of second-phase thermoplastic particles.
The article, "The Toughening Effects of PBI in a BMI Matrix Resin", M. T. Blair, et al, 33rd International SAMPE Symposium, March 7-10, 1988, page 524, notes that second-phase thermoplastics, such as polysulfone, are blended into epoxy resin systems for toughening the epoxy-based resin. This paper then applies this method to a BMI matrix resin, employing polybenzimidazole as thermoplastic.
The article, "Semi-IPN Matrix Systems for Composite Aircraft Primary Structures", G. R. Almen, et al, 33rd International SAMPE Symposium, March 7-10, 1988, page 979, examines the addition of polyaromatic thermoplastic to epoxy resins.
However, the systems of the above articles are generally too brittle and do not provide tough composites having superior damage tolerance characteristics.
Examples in the prior art relating to polymers, such as thermosetting or thermoplastic resins modified by the use of various fillers, are set forth in the patents below.
Patent No. 4,757,120 to Bristowe, et al, discloses a polymer blend of a bismaleimide derivative with 2 to 15% by weight of a polyethersulfone, stated to be useful in producing prepregs and filled composites, particularly useful in structural components having high stability at high temperatures.
Patent No. 4,468,497 to Street, et al, discloses a matrix resin system for graphite fiber composites comprised of a bismaleimide and polyethersulfone.
Patent No. 4,749,760 to Wang discloses a curable resin composition comprising a dicyanate-terminated aromatic polysulfone oligomer and a bismaleimide component.
Patent No. 3,920,768 to Kwiatkowski discloses arylimide-epoxy resin composites. In preparing such composites, arylimides prepared from oligomer diamines and polyamines and maleic anhydride are blended with epoxy resins which can be cured to thermoset composites.
Patent No. 4,562,231 to Dean discloses moldable polymer compositions comprising a polyethersulfone resin and a copolymer containing recurring units of a vinyl aromatic monomer and recurring units of a maleimide monomer
Patent No 4,766,179 to DeKoning discloses a bismaleimide composition comprising a bismaleimide compound and including an alkylacrylate component.
Patent No. 4,144,284 to Semanaz, et al, discloses shaped articles consisting essentially of a matrix of a thermoplastic polymer, such as polyethylene or polypropylene, in which there is dispersed, in the form of particles of mean diameter less than 200 microns, a polyimide resin.
Patent No. 4,212,959 to Fukami, et al, discloses a heat resistant resin composition comprising a mixture including a maleimide component and an epoxy resin.
Patent No 4,691,025 to Domeier, et al, discloses bismaleimides and prepreg resins therefrom comprising bismaleimide and optionally a thermoplastic polymer, such as a polysulfone.
However, none of the above patents is directed to the concept of increasing damage tolerance in thermoset composites by incorporation of thermoplastic fillers through controlled solubility of such fillers in the thermoset.
One object of the present invention is the provision of thermoset composites having increased damage tolerance without loss of processibility or mechanical properties.
Another object is the provision of a process for the addition of thermoplastics to thermoset matrix resins without changing the processing and physical characteristics of the thermosets.
Still another object is to provide procedure for the incorporation of relatively large amounts of a thermoplastic resin in a thermoset resin matrix by controlled solubility of the thermoplastic filler in the thermoset, so that upon curing of the system, a substantially homogeneous material comprised of the two resins is obtained, having improved composite damage tolerance and good thermal stability.
Yet another object is the provision of thermoset-thermoplastic compositions which can be converted into pre-pregs and composites having enhanced damage tolerance as compared to the thermoset resin per se, and good thermal stability and mechanical properties, and to the provision of the improved pre-pregs and improved composites so produced.
Other objects and advantages of the invention will be apparent from the description below of the invention.